In the manufacture of pressure-sensitive recording papers, better known as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is normally coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated backside is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of a suitable color developer, also known as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor.
Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff, or dyestuff precursor, then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such mechanism for the technique of producing pressure-sensitive recording papers is well known.
Among the well known color developers used on CF record sheets are phenolic-type resins, such as acetylated phenolic resins, salicylic acid modified phenolics and, particularly, novolac type phenolic resins.
Among the well known basic, reactive, colorless chromogenic dye precursors useful for developing colored marks when and where applied to a receiving sheet coated with such color developers are Crystal Violet Lactone (CVL), the p-toluenesulfonate salt of Michler's Hydrol or 4,4'-bis(diethylamino)benzhydrol, Benzoyl Leuco Methylene Blue (BLMB), Indolyl Red, Malachite Green Lactone, 8'-methoxybenzoindoline spiropyran, Rhodamine Lactone, and mixtures thereof.
Microencapsulation has been used in the production of carbonless copy papers for some time. One of the major techniques involves phase separation from an aqueous solution. The complex coacervation process (U.S. Pat. No. 2,800,457 and others) falls into this category. In such a process, a phase separation into a liquid condensed colloid phase and a dilute colloid phase results from two oppositely charged condensed colloids neutralizing each other. Under appropriate conditions, the condensed colloid phase can be induced to first surround and envelope the oil droplets, and then be hardened to form the microcapsules.
Polymer insolubilization by pH adjustment is another implementation of this general technique. The polymer must become insoluble in water within a specific pH range. Casein is a high molecular weight (33,600-375,000) globular protein with pendant amino, hydroxyl, and carboxylic acid groups, and as such possesses this property. Alone, the polymer is essentially insoluble in water, and this insolubility is at a maximum at its isoelectric point of pH 4.6. However, through salt formation with the amino and carboxylic acid groups, casein will dissolve in bases and in strong acids pH 8, pH 3). Thus, the solubility of casein in water can be controlled by pH adjustment. This property has been used in the prior art to encapsulate oils and solids. Japanese patents 37/7727, 37/7731, and 37/9681, disclose the microencapsulation of dye solutions by precipitation. The dye solution is first dispersed into an alkaline solution of casein wherein the casein (actually sodium caseinate) acts as a polymeric emulsifier. Then, the pH is gradually decreased to 4.6, causing the casein to precipitate onto and envelop the droplets. The resulting capsules were stabilized and strengthened/hardened by insolubilizing the casein with formaldehyde or by drying the capsules through spray drying. A similar preparation of capsules is disclosed in French Patent 1470724. In all cases, as a result of the chemistry of aldehyde hardeners, a structurally strong capsule that can withstand moderate amounts of shear and pressure commonly incurred during handling was not achieved until the capsules were completely dry.
This encapsulation technique has several disadvantages:
(1) The precipitation of casein as a solid does not uniformly occur at the interface of the oil droplet. The nucleation and growth of the solid occurs in the bulk of the water, creating particles of casein of varying size, some of which may or may not collect at the oil-water interface. The result is a non-uniform layer of casein around the droplets. This capsule can have varying thicknesses around the circumference, with numerous cracks or holes in coverage. As a result, the capsules have a great tendency to leak, producing discoloration when used in carbonless papers, and limited shelf life.
(2) The capsules are extremely fragile, and must be spray dried before they are sufficiently strong to withstand normal handling. Thus, they are essentially useless in applications where strong wet capsules are required, as in the case of applying them to paper.
(3) The process uses a high casein:oil ratio, and must be performed at a low solids content (10%) in order to maintain a workable viscosity when the casein is precipitated. This makes subsequent coating or processing (drying) expensive due to the large amounts of water that must removed.
In response to these shortcomings, interfacial polymerization or interfacial crosslinking was developed. In U.S. Pat. No. 4,138,362, an amine containing polymer is crosslinked or polymerized at the interface by a polyisocyanate dissolved in the dye solution, which is emulsified in the polymer. In the case of casein, the capsules produced by this method are extremely poor. The capsules are very fragile, due to a very thin wall, and exhibit poor aging as a result of a steady release of the solution they contain.
This problem apparently stems from (1) the colloidal nature of casein solutions (they exhibit the Tyndall effect), (2) the high molecular weight of casein, and (3) the anionic (charged) nature of sodium or alkali metal salt solutions of casein. The alkali salt of casein functions as a high molecular weight polyanionic emulsifier and protective colloid. When an oil is emulsified in an alkaline casein solution, a thin layer is formed around the droplet. The polyisocyanate can crosslink this small amount of casein absorbed on the surface, but additional casein cannot reach the isocyanate. The very properties that make sodium caseinate a good emulsifier and protective colloid inhibit further reaction. Namely, the negative charge around the droplet created by the thin molecular layer creates the familiar double layer that functions as a barrier to further penetration of ionized casein to the droplet surface. The high molecular weight also sterically hinders the diffusion of additional casein to the reaction zone near the oil-water interface. The fragile capsules produced by the simple interfacial polymerization/crosslinking are inadequate for the processing procedures needed for preparing carbonless papers, namely, filtration and dispersion into ink vehicles.
Another method to improve the strength of capsule walls is shown in U.S. Pat. No. 4,404,251, which discloses printing ink containing microcapsules containing dye precursors. The microcapsules are made by polyaddition of a polyisocyanate and a polyamine, and the aqueous phase may contain protective colloids and emulsifiers. The formed microcapsules are formulated into the printing ink composition by stirring the aqueous dispersion of microcapsules directly into the binder and subsequently removing the water in vacuo. Alternatively, the microcapsules may be spray dried and then added to the binder. Dispersing aids, preferably cationic surfactant, may be added to the ink to prevent agglomeration of the microcapsules.
Reference is also made to U.S. Pat. No. 4,193,889 which discloses microcapsules and a process for the production of microcapsules the walls of which consist of polycondensates of a film-forming aliphatic polyisocyanate containing at least one biuret group, or polyaddition products thereof, with a chain extending agent. The chain extending agent is preferably either water, a polyol or a polyamine. It is stated in that patent that the so-produced microcapsules have improved toughness, show adequate crosslinking density, and, therefore, are only slightly permeable to easily volatile encapsulated substances.
Another method for dealing with the problems of fragile microcapsules is disclosed in U.S. Pat. No. 4,435,340, wherein an isocyanate is used in the hydrophobic phase and a polyamine, such as a low molecular weight polyamine, is used in the hydrophilic phase. Microcapsules are formed by interfacial polymerization.
U.S. Pat. No. 4,356,108 discloses an encapsulation process by interfacial reaction of an isocyanate and a low molecular weight polyamine.
Even with these various improved microcapsules, processes for printing microcapsules in coating compositions on offset printing machines or even book printing machines were heretofore regarded as unworkable because both in the production of the printing ink and in the distributor rollers of the printing machine and during the printing process, shearing and compressive forces would destroy most of the microcapsules. A process for accomplishing such printing inks is described in U.S. Pat. No. 4,404,251 in which formed microcapsules are formulated into the printing ink composition either by stirring the aqueous dispersion of microcapsules directly in the binder and subsequently removing the water in vacuo (the so-called flushing process), or by spray-drying the microcapsules and then adding to the binder. These processes require special equipment and are not entirely satisfactory. In the former process, the hydrophilic nature of the microcapsules may make direct incorporation into the binder very difficult. The spray-drying technique is very costly. Furthermore, during spray-drying some capsules inevitably aggregate which results in a large particle size distribution. The aggregates can easily reach 100 microns or more, and once formed are virtually impossible to break up non-destructively to the capsule. Such large particles are quite unsuitable for inks.
Accordingly, the need remains for improved microcapsules which are sufficiently strong to be dispersed in an ink vehicle and press applied.